This invention relates to transistors, both hot-electron transistors and hot-hole transistors, particularly but not exclusively for high frequency applications, fast switching applications, or opto-electronic applications. Such a transistor may be in the form of a discrete device, or it may be integrated in a more complex structure, for example in a monolithic integrated circuit.
Published U.K. patent application serial no. (GB-A) 2056166 discloses both hot-hole transistors and hot-electron transistors comprising a semiconductor body including a silicon surface region adjacent a surface of the body, a base region which at least partially underlies the surface region, a collector region which at least partially underlies the base region, and barrier-forming means which form an emitter-base barrier between the base region and at least a portion of the surface region and a base-collector barrier between the base region and the collector region, the current flow through the base region from across the emitter-base barrier being by hot charge carriers characteristic of one conductivity type. Such transistors are majority-carrier (unipolar) devices and so can be operated at a fast speed or high frequency. The surface region may be of the one conductivity type and form part of the transistor emitter, or it may be a low-doped region of opposite conductivity type and form part of the emitter-base barrier.
In one form of transistor disclosed in GB-A 2056166, areas of the semiconductor body (which is of gallium arsenide) are etched away to remove corresponding areas of various regions of the transistor over the whole of their thickness so as to terminate laterally the emitter-base and base-collector barriers in an isolated manner at the semiconductur surface and to expose the individual regions of the transistor for contacting at the surface by respective electrode layers. However such etching results in an unpassivated non-planar mesa-shaped semiconductor surface, whereas a passivated substantially planar surface is generally more desirable, particularly for integrated circuits.
Most of the transistors disclosed in GB 2056166, as well as in the related U.S. Pat. No. 4,149,174 to which reference is made in GB 2056166, have lateral portions of the individual semiconductor regions of the transistor which extend to the upper body surface in a laterally surrounding manner. Sometimes a highly-doped extra surface-adjacent region is provided to form these lateral portions. The upper surface of the semiconductor body (which is of silicon) is substantially planar and is passivated with an insulating layer. Thus it is known in these transistors to terminate the base-collector barrier and the emitter-base barrier in an undepleted laterally-surrounding annular region of opposite-conductivity type to that of the hot charge-carrier flow in the base region. This undepleted opposite-conductivity type region also provides p-n junctions, which serve for isolating from each other the collector, base and emitter-surface regions adjacent the upper surface where electrode connections are made to the individual regions of the transistor. It has been found that such p-n junction isolation can lead to slight minority charge carrier storage effects in these majority carrier devices, as well as being wasteful of space at the surface of the semiconductor body.